Semiconductor wafer fabrication involves many complex and highly precise processes. Many of these processes are highly dependent on the precise control of process temperature. Small variations in temperature can have detrimental effects on the resulting semiconductor devices. One such fabrication process is chemical vapor deposition. Chemical vapor deposition (CVD) is a widely used process for depositing thin films of a variety of materials. In addition to semiconductor electronics, applications of CVD include the deposition of protective coatings for other applications (e.g., optics, mechanical parts, etc.).
In a typical CVD process, a mixture of reactant gases (often diluted in a carrier gas) at room temperature are injected into a CVD reaction chamber. The gas mixture, as it approaches a deposition surface (e.g., a semiconductor wafer), is heated radiatively by thermal lamps, or alternatively, placed upon a heated substrate. Depending on the precise process temperature and operating conditions, the gas mixture typically undergoes homo geneous chemical reactions in the vapor phase before striking the deposition surface. Near the surface, thermal, momentum, and chemical concentration boundary layers form as the gas stream heats, slows down, and the chemical composition changes. Heterogeneous reactions of within the gas mixture subsequently occur at the deposition surface, forming the deposited material (e.g., thin film). The resulting reaction by-products are then transported out of the CVD reaction chamber.
The characteristics and the results of the CVD process very much depend on controlling the process temperature. High temperatures are often used (e.g., 280 C.). The precise operating temperature within the CVD chamber is typically regulated and maintained by an embedded computer system within the CVD machine. This computer system implements a software defined process for heating the CVD chamber, following a temperature profile during processing, and protecting the components comprising the CVD machine from being damaged by excessive heat.
There is a problem, however, in that under certain conditions, errors can occur with the embedded computer system and its software. The computer systems of CVD machines and their controlling software are thoroughly tested and exhaustively examined prior to use of the machines in a fabrication line. As such, any errors which may occur are usually of little or no consequence, since a typical CVD machine includes a variety of error handling routines to diagnose and fix such errors. On occasion, however, an error of sufficient severity may occur which results in the scrapping of wafers undergoing processing. Even worse, under certain conditions, a catastrophic error may occur which results in damage to the CVD machine itself. One such catastrophic error is thermal "lock up".
Thermal lock up refers to a condition where the embedded computer system controlling the CVD machine malfunctions and looses the ability to shut off heating components (e.g., radiant thermal lamps within the CVD chamber). For example, in the case where a CVD machine is processing a wafer and following a process temperature profile, the embedded computer system modulates the heating elements, alternately turning them on and off, to achieve and maintain a desired temperature within the CVD chamber. If a malfunction occurs during a period when the heating elements are "on", the embedded computer system may loose the ability to subsequently turn them "off". For example, if the embedded computer system "locks up" (e.g., due to a software error or a power supply voltage glitch) after having commanded the heating elements on, the command to turn the heating elements off may not be issued. Consequently, the temperature within the CVD chamber "runs away," increasing to the point at which some components within the CVD chamber are significantly damaged.
The cost of repairing the CVD machine can be very high. A modern CVD machine is an extremely accurate, complex device. In addition to the costs of repairing the CVD machine itself, however, are the costs associated with the machine's lost productivity. A typical wafer fabrication line involves the production of hundreds, and perhaps thousands, of semiconductor components daily. The interruption in production could be much more costly than the cost attributed to repairing of the CVD machine.